This invention relates to a porous sheet material having a chemical agent applied thereto for the purpose of releasing the agent into the soil on a gradual or long-term basis.
Various methods have been proposed for treating soil for controlling or regulating the growth of plant or animal life. If the chemical agent is applied directly to the soil, however, the effects are temporary, since the chemical tends to be washed away or degraded. A particular serious problem is the long term control of plant root growth in certain locations such as beneath remote power transmission lines or into areas containing toxic materials. Many herbicides are useful for these purposes, including those described in U.S. Pat. Nos. 3,111,403, 3,257,190 and U.S. Pat. No. 4,101,582. A particularly useful herbicide is N,N-di-n-propyl-4-trifluoromethyl-2-6-dinitroaniline, which is known by the generic name trifluralin and sold under the trademark "TREFLAN."
In order to accomplish long term control of plant growth and root development, it is known to provide slow release systems for herbicidal and other soil treatment compounds. The herbicide or other compound may be incorporated or dispersed in a solid matrix of a water insoluble polymer such that the active compound is released very slowly, i.e., for periods for up to one hundred years. These slow release materials are typically prepared in the form of solid pellets which are deposited in the soil in a uniformly spaced manner. Manually positioning the individual pellets in the required pattern in the soil is time consuming and cost prohibitive. In addition, if there is nothing to hold the pellets in place, however, they tend to become displaced over a period of time, causing loss of control. Passage of water or soil disruption may cause movement of the pellets to adjacent areas, causing damage to useful plants or crops.
A geotextile is a porous sheet material which may be applied on or beneath the soil surface for a variety of purposes, for example, to block passage of sunlight or to control erosion, while allowing passage of water. These textiles are usually made from non-degradable materials such as polymers. Particular examples included non-woven spunbonded textiles made from filaments or fibers of polypropylene, such as products sold under the trademark "Typar." It would be desirable to incorporate slow release soil treating agents into geotextiles for the purpose of selectively treating soil in specific locations.
In order to be effective, the slow release material must have a certain minimum size or cross section to provide a minimum length herbicide migration path. A second requirement is the precise relative positioning of the deposits in the soil to obtain a durable and impenetrable root barrier plane. Thus, for example, direct incorporation of a herbicide into the geotextile during manufacture or by applying a uniform coating onto a porous substrate is either not feasible or would not provide the required slow release feature.
A preferred slow release system comprises a herbicide such as trifluralin adsorbed into carbon black and dispersed in a matrix of polyethylene. An attempt was made to place uniform deposits of this mixture onto a spunbonded web of polypropylene. It was found that the herbicides generally have a much lower melting point than the time release polymer, causing loss of herbicide during application of the mixture in molten form. Also, it is very difficult to make the mixture adhere to the porous web even with the use of adhesives.